1. Field of the Invention
The invention relates to production of complex components from metallic or ceramic materials wherein powders are used as the starting material.
The invention relates to the further development, perfection and simplification of powder-metallurgical production methods for the production of workpieces of comparatively complex shapes, where the problems of shrinkage during sintering play an important role. The field of application is, in particular, the component sector in turbine construction.
In the narrower sense, the invention relates to a process for the production of a component of arbitrarily complex shape from a metallic or ceramic material or a plastic by production of a dense-packed molding using a pourable powder or a powder mixture as the starting material, by blowing the powder, fluidized by a transport gas, into a negative mold which is under reduced pressure and further treatment of the pre-compacted body, by subjecting the body to a sintering process. The invention also relates to an installation for carrying out the process.
2. Discussion of Background
Powders are used as the starting materials in numerous production methods in the metallurgical and ceramics industry. The intention is to produce workpieces by powder metallurgy as finished articles in order to be able to save on some or all expensive machining costs. The starting materials in all of the known processes for obtaining net shapes or near-net shapes of the workpieces are slurries (slip, paste) of powders in solvents using a binder. The following are used as additives to powder mixtures:
water+binder+additive (slip casting, freeze drying) PA0 water+cellulose (metal-powder injection molding (MIM) by the Rivers process) PA0 thermoplastics (metal-powder injection molding). PA0 bubble formation when mixing powder with binder and solvent. PA0 Restriction of the wall thickness of the workpieces (for example max. 5-10 mm for "MIM"), since otherwise the binder can no longer be completely removed. PA0 The occurrence of binder residues (for example carbon), which, even after "burning out" the binder, remain behind in the workpiece and can impair its composition in an uncontrolled manner. PA0 The necessity for fresh selection/fresh development of the binder when changing to other shapes and/or compositions of the workpieces. PA0 GB Pat. Appl. 2088414 PA0 EP Pat. Appl. 0191409 PA0 DE-A-3,101,236 PA0 DE-A-3,128,347 PA0 DE-A-3,128,348 PA0 DE-A-3,542,332 PA0 R. Billet, "PLASTIC METALS: From Fiction to Reality with Injection Molded P/M Materials", Parmatech Corporation, San Rafael, Calif., P/M-82 in Europe Int. PM-Conf. Florence I 1982. PA0 Goran Sjoberg, "Powder Casting and Metal Injection Molding", manuscript submitted to Metal Powder Report September 1987 PA0 Henry H. Hausner, "Slip Casting of Metal Powders", in "Perspectives in Powder Metallurgy", Hausner et al., Plenum Press 1967 PA0 introduction of a negative mold, which determines the shape of the component and consists of a porous material which is permeable to gas but not permeable to the powder, at least in the surface zone of the inner wall, into a container which is under reduced pressure, uniform gas flow conditions being provided for applying the powder to the inner wall by centrifugal force, PA0 blowing the powder into the cavity of the negative mold using a transport gas which is under high pressure, a green strength which is adequate for the further treatment of the green compact being achieved, PA0 flooding the container in order to produce pressure equalization in the negative mold, removing the negative mold together with the green compact from the container for further treatment, and PA0 sintering the green compact.
With all of these wet-mechanical methods numerous difficulties arise with regard to quality, freedom of shaping, reproducibility and choice of the composition:
In the case of metal injection molding (MIM) a mixture of the metal powder to be compacted is injected into a mold together with a suitable thermoplastic in accordance with the injection molding technology. A summary of the methods for "Metal Injection Molding" is given in a chapter of the Metals Handbook.
A particular problem with this technology is, on the one hand, the fact that in general considerably finer powders have to be employed than is usually the case in powder metallurgy; on the other hand, the organic binder must be removed by a laborious process before the actual sintering process, which leads to a considerable increase in the cost of the process.
The vacuum-molding process, which serves for the production of casting molds from refractory granular mold material, as a rule quartz sand, is known from casting technology. By evacuation of the air from a heap of binder-free sand surrounded by sheeting, a reduced pressure is generated in said sand, as a result of which a compressive pressure is exerted by the adjacent outside gas atmosphere via the sheeting on the sand fill. The compressive strains caused by this means between the grains prevent the mutual mobility of the latter, as a result of which a mechanically strong body of defined shape is formed from a loose heap.
In the production of moldings which are subjected to a subsequent sintering, the uniformity of the powder fill at all points of the molding is extremely important since the local extent of shrinkage, and thus the dimensional accuracy, are a function of the local settled apparent density.
There are processes from the field of powder metallurgy where mixtures of a metal powder with a liquefied organic phase are injected into molds by the injection molding process. After the filling operation, a compact composite of uniform density is formed, from which the organic binder must be removed before the actual sintering process starts.
There are other processes in which essentially dry powder is filled into a mold under vacuum. This operation can, for example, also be supported by a suitable vibration or shaking operation. However, because of the frictional resistance of the powder, there are limits to the complexity of shaping, or there is a risk that the various grain fractions of a powder will demix under the influence of the movement of the powder, especially under the action of vibrations, as a result of which an inhomogeneous sintered compact forms.
With the aid of one process, for example, a molding is produced by a procedure in which a pourable molding composition is fluidized using a transport gas and in this way passes into the interior of a mold which is under reduced pressure and which contains suction orifices at certain points for drawing off the transport gas. A substantial part of the description of this process is dedicated to the optimum sizing and arrangement of these suction orifices and to the optimum timing of the injection and suction processes, since both the geometrical arrangement and the timing are of extremely great importance for the production of a molding having a uniform settled apparent density. When the fluidized powder penetrates into the interior of the mold expansion of the gas, and thus kinetic acceleration of the powder particles, occurs, which powder particles are driven by centrifugal force against the wall of the mold. Since, however, the wall of the mold is impermeable to gas in substantial sections, only a coating of the wall is achieved by the kinetic energy of the grain particles. Special precautions must, in particular, be taken in order to prevent premature blocking of the off-gas channels by powder preferentially flying in this direction.
The following publications are cited in respect of the prior art:
The known processes leave something to be desired. There is therefore a need for improvement and further development of the powder-metallurgical/powder-ceramic production methods.